When the power is turned on for a power supply device, there may be an inrush current that flows in the power supply device when a smoothing capacitor is charged. To prevent the inrush current from damaging an element, the power supply device adopts a configuration in which a resistor is connected in series with the smoothing capacitor, thereby suppressing the inrush current when the power is turned on.
In Patent Literature 1, such a power supply device is disclosed in which a relay is connected across an overcurrent protection circuit that includes an inrush-current suppressing resistor. When the power is turned on, the relay is opened and the charging current of the smoothing capacitor is consumed as heat by the inrush current limit resistor, thereby suppressing the inrush current when the power is turned on. Upon completion of charging the smoothing capacitor, the power supply device disclosed in Patent Literature 1 closes the relay and isolates the inrush-current suppressing resistor, thereby causing the current to flow to the relay without bypassing the inrush-current suppressing resistor.
Patent Literature 2 discloses a power supply device in which, when a momentary cutoff of the power supply occurs due to an instantaneous power failure, the voltage across the smoothing capacitor is compared with a set protection voltage, and the relay connected in parallel with the inrush-current limiting resistor is controlled on the basis of the comparison result, thereby suppressing the inrush current.